1. Field of the Invention
The present invention relates to a pulley which is utilized in conjunction with a V-shaped belt. More specifically, the invention relates to a pulley which releases its grip on the belt when a predetermined torque is exerted on the pulley.
2. Description of the Prior Art
Pulleys are frequently utilized to transfer rotational movement from one shaft to a second shaft. A pulley is mounted on each of the two shafts and a flexible belt is suspended therebetween. The rotation of the first shaft is transferred by the belt to the second shaft which is in turn rotated. The size ratios of the pulleys are generally determined by the need to increase or decrease the speed or torque of the driven shaft relative to the driving shaft. As the speed of the belt increases, the danger of a thrown belt or jammed shaft also increases. This danger is also increased because modern belts are frequently impregnated with metal strands to increase their strength.
The primary cause of a broken or thrown belt is a sudden counterforce acting upon the spinning shaft or pulley. For example, an electric motor having a spinning shaft is provided with a driving pulley. A belt extends to a second pulley mounted on a second shaft which drives a drill bit. As the drill bit begins cutting, a counterforce is exerted on the bit and back through the system by the friction of the bit on the cutting surface. So long as this force does not exceed the frictional forces holding the belt in registration with the pulleys, the system continues to operate normally. If, however, the drill bit encounters a material of greater hardness than previously encountered, or the like, the drill bit may be suddenly stopped or slowed. In this example, the drill bit, its attached shaft and the associated pulley, would suddenly stop or slow in a like manner as the torque in the shaft is increased. The electric driving motor and its associated shaft and pulley, however, would continue to run normally. This produces frictional forces on the belt connecting the two pulleys and generally tends to tear the belt.
Several devices have been proposed in the past to provide a release mechanism for one or both of the pulleys, such that when the counterforce from the driven shaft is encountered by the pulley, the belt is allowed to turn independently of the shaft which has encountered the restraining force. One type of such a device is exemplified by Smith, U.S. Pat. No. 2,604,765, issued July 29, 1952. Smith provides a pulley which may freewheel on a shaft. A clutch mechanism is attached to the shaft in the form of a circular frictional plate which is adapted to engage an interior mating surface on the pulley. The frictional plate is spring loaded. The frictional plate is in the shape of a broken circle, and is mounted inside the circular pulley. The C-shaped spring mechanism is also circular and tends to force the friction plate into a circle having a larger circumference. This provides a frictional contact between the friction plate and the pulley. When the rotational force on the pulley become greater than the frictional force on the spring urged friction plate, the pulley slips with respect to that plate.
A second type of torque override pulley is illustrated by Dahlstrand et al., U.S. Pat. No. 3,132,730, issued May 12, 1964. Dahlstrand et al. utilizes a slidable key to connect a driving element and a driven element. When the torque on the driven element rises above a preset limit, the key is forced away from a mating surface, releasing the pulley from the driving force. The key is held in place by a spring loaded lock which determines the amount of torque necessary to move the key away from the mating surface.
A third type of pulley is illustrated by Gebert, U.S. Pat. No. 2,255,211, issued Sept. 9, 1941. Gebert utilizes a pulley which spins freely on a shaft. The shaft has a cammed surface attached thereto which engages a spring loaded lever attached to the pulley. The cam and the lever move in registration with respect to each other so long as the force from the torque on the pulley does not overcome the force of the spring which biases the lever against the cam. When the preset torque is exceeded, the cam shifts the lever against the spring into a disengaged position.
Dunihoo, U.S. Pat. No. 2,842,005, issued July 8, 1958 discloses a clutch mechanism which utilizes ball bearings riding on a cam surface as the clutch device. In the operative mode, the driving shaft is engaged to the pulley hub through ball bearings. The ball bearings ride in a groove which has a cammed surface. When the torque becomes too great, the ball bearings are forced against the cam surfaces, which then moves the ball bearings away from the driving surface. This disengages the hub and the driving shaft.
The above described devices have several limitations. All have a wide variety of moving parts which must be carefully machined to specific tolerances to allow for continued operation. Additionally, this large number of component pieces increases the cost and time required for the assembly of the device. Furthermore, some of the devices require manual resetting once the safety feature has been activated. What is lacking in the art, therefore, is a simple, automatic resetting pulley which allows the belt to operate normally under normal torque conditions but to disengage without damage when a sudden high torque is encountered. The device should be of simple construction and easily manufactured and assembled.